The partial oxidation of hydrocarbons is a very important chemical reaction, and may rank as one of the most difficult of organic transformations. Nature provides partial oxidation of hydrocarbons via the monoxygenase enzymes of the cytochrome P450 family, which generally process xenobiotics to generate hydroxylated metabolites. The reaction utilizes oxygen and a reducing agent to effect alkane oxidation, and involves an intermediate including an iron porphyrin group.
The desire to create synthetic systems for carrying out partial oxidation of hydrocarbons has generated substantial research, and significant developments have been achieved in areas including olefin epoxidations, alkane hydroxylations, oxidations of pollutants, drugs, sulfur-or nitrogen-containing molecules, and the like, as reviewed by B. Meunier in "Metalloporphyrins as Versatile Catalyst for Oxidation Reactions and Oxidative DNA Cleavage", Chem. Rev., 92, 1411-1456 (1992). Metalloporphyrins have been effectively utilized in the catalytic oxidation of hydrocarbons to produce a variety of valuable fine chemicals.
One example of a partial oxidation reaction is that of alkene to epoxide involving iron porphyrin as a catalyst. In the reaction, four of six available iron coordination sites of the iron porphyrin complex are occupied by the essentially planar porphyrin ring, and one of the free remaining iron coordination sites, which faces out of the plane of the porphyrin ring, is involved in the oxidation reaction, demonstrating the need for accessibility of the side of the porphyrin ring to reactants. In particular, oxygen transfer to the metal center from an oxidant, followed by association of a hydrocarbon with the oxygen at the metal center and subsequent partial oxidation, is required.
Synthetic metalloporphyrins that lack nature's globin protection must maintain physical separation of the porphyrins' active metal sites to avoid self-oxidation. If not, during the catalytic reaction, monomeric oxo-metal adducts can be converted to .mu.-oxo dimers by auto-oxidation and catalytic activity can be lost. This demonstrates the need for efficient heterogeneous catalysis systems, that is, systems in which a solid phase carrying a catalyst is contacted with a fluid phase containing reactant and oxidant. In such systems catalytic metal centers can be maintained in isolation from each other, and the catalyst can be easily recovered. Because of the importance of catalysts involving metal porphyrins, substantial efforts have been devoted to development of solid-phase-supported metalloporphyrin catalyst systems. Description of some of these efforts can be found in the following references.
Campestrini and Meunier, in "Olefin Epoxidation and Alkene-Hydroxylation Catalyzed by Robust Sulfurated Manganese and Iron Porphyrins Supported on Cationic Ion-Exchange Resins", Inorg. Chem., 31, 1999 (1992) describe immobilization of metalloporphyrins to polymers either via direct attachment of the metal to the polymer by coordination of a pyridine unit on the polymer to the metal center, by electrostatic attraction of functional groups such as SO.sub.3.sup.- on the porphyrin ring with functional groups such as NR.sub.4.sup.+ on the polymer, or both.
U.S. Pat. No. 5,141,911 (Meunier, et al.) describes metalloporphyrins carrying anionic groups as substituents in which the metalloporphyrins are immobilized on a support which is made of a polymer containing nitrogenous groups used as a Lewis base.
Barloy, et al., in "Manganese Porphyrins Supported on Montmorillonite as Hydrocarbon Mono-oxygenation Catalysts: Particular Efficacy for Linear Alkene Hydroxylation", J. Chem. Soc., Chem. Commun., 1365 (1990) describe immobilization of manganese porphyrin complexes on clay. The mode of binding of the complex to the clay is not fully characterized, although UV-visible spectroscopy shows two intense peaks at 471 and 496 nm, corresponding to two different environments of manganese. No peak corresponding to the free base porphyrin was observed.
Battioni, et al., in "Mono-oxygenase-like Oxidation of Hydrocarbons Using Supported Manganese-Porphyrin Catalysts: Beneficial Effects of a Silica Support for Alkene Hydroxylation", J. Chem. Soc., Chem. Commun., 1149 (1989), described manganese porphyrin adsorbed on silica, alumina, and magnesia.
N. Herron, in "The Selective Partial Oxidation of Alkenes Using Zeolite Based Catalysts. Phthalocyanine (PC) `Ship-in-Bottle` Species", J. Coord. Chem., 19, 25 (1988) describe iron phthalocyanine complexes immobilized within pores of zeolites.
Liu, et al., in "Hydroxylation of phenol by iron(II)-phenanthroline(Phen)/MCM-41 zeolite", Catalysis Letters, 263-266 (1996) describe immobilization of iron (II)-phenanthroline within the hexagonally-packed cylindrical pores of MCM-41 silica. Liu, et al. state that immobilization may be due to adsorption and static coulombic interactions between the surface of MCM-41 and the iron (II)-phen species.
Leal, et al., J. Am. Chem. Soc., 97, 5125 (1975), describe covalent attachment of metalloporphyrins to polymers.
U.S. Pat. No. 5,274,090 (Zhang, et al.) describes covalent attachment of a crown ether to two diagonally opposing phenyl groups of a metalloporphyrin, and a molecular bridge covalently linking two diagonally opposing phenyl groups on the side of the porphyrin opposite the crown ether effective to hinder .mu.-oxo dimer formation.
While advances in immobilization of metal porphyrins on surfaces have been made, most systems are not optimal. Many prior art systems involve non-covalent interaction between catalyst and support (ionic or static interaction), which can be weaker than desirable and can result in leaching of the catalyst from the surface over time. Some systems involve catalyst immobilization via interaction of the catalytic metal center with the surface which, while effective in some circumstances, can in some cases hinder the metal's participation in catalysts. Some systems use solid phase support that have small pores that can cause deformation of the metal catalyst, potentially adversely affecting catalytic performance, and which may not allow sufficient access by reactants. Polymers covalently linked to catalytic systems do not offer the structural advantage of inorganic solid phases, which can self-assemble as mesoporous supports that provide high surface areas, that define uniform channels forcing reactants to be brought into close proximity with immobilized catalyst, and that can offer selectivity in terms of both reactants and products. In addition, in some instances it can be advantageous to conduct a reaction involving an immobilized catalyst at a temperature above the degradation temperature of some polymers.
It is, therefore, an object of the invention to provide a catalyst including a catalytic metal atom immobilized at a surface in a manner providing long-term, high-capacity operation, and allowing easy access of reactants to the catalytic metal atom.